The present invention relates to composite moulded refractory articles and their manufacture.
Articles with which this invention is concerned are such items as refractory bricks, well blocks, nozzles, valve plates and parts and fittings therefor, which molten metal streams contact in the course of teeming.
A recurrent problem in the teeming of molten metal, e.g. steel, is the erosion of refractories contacted by the flowing metal. The refractories include those forming the discharge outlet region of a ladle or tundish lining, and the plates and collector nozzles or pour tubes of sliding gate valves. Another problem, when pouring certain alloy steels such as Al-killed steels, is accretion of frozen metal and alumina on the refractories.
Hitherto, the aforementioned problems have been tackled by making the vulnerable parts from very costly high-temperature fired refractories. Usually, high alumina is chosen. For components especially likely to be degraded, e.g. the refractory parts of throttling valves, even more expensive zirconia inserts or liners have sometimes been incorporated in the refractory parts owing to the high resistance of zirconia to molten metal attack.
It has been shown for slide gate valve plates that, apart from surface zones most adjacent the areas which come into contact with molten metal, the plate temperatures are usually well below 1000.degree. C. Nevertheless it has been usual to make valve plates in entirety from refractories capable of withstanding very much higher temperatures. This is unnecessarily wasteful. Much lower duty and cheaper refractories would be quite adequate except adjacent metal contact regions. It is believed other refractory items could be made more economically if the bulk thereof were made from lower duty refractories while their metal contact surfaces are made from higher duty refractories.
Further it may be mentioned that, as regards sliding gate valve plates, in general no more than 40% or so of their sliding surface areas are ever likely to be exposed to molten metal. No more than 25% or so of their volumes is likely to be exposed to temperatures above 1000.degree. C. Thus, only a limited quantity of higher duty refractory is actually necessary for a sliding gate valve plate to perform satisfactorily.